Method for producing stainless steel

ABSTRACT

THIS INVENTION RELATES TO A STAINLESS STEEL AND A METHOD FOR PRODUCING THE SAME, WHEREBY MATERIAL PARTICULARLY ADAPTED FOR USE IN STRUCTURAL APPLICATIONS SUCH AS THE MANUFACTURE OF CARGO BOXES IS ACHIEVED. SPECIFICALLY, THE MATERIAL IS CHARACTERIZED BY AN IMPROVED COMBINATION OF STRENGTH AND TOUGHNESS THAT IS ACHIEVED BY PRODUCING HOTBAND MATERIAL HAVING A SUBSTANTIALLY MARTENSITIC MICROSTRUCTURE OF A COMPOSITION CONSISTING OF .10 MAX. PERCENT CARBON, 2 MAX. PERCENT MANGANESE, 1 MAX. PERCENT NICKEL, 9.5 TO 13.5 PERCENT CHROMIUM, AND THE BALANCE IRON. THIS MATERIAL HAS A MAXIMUM TITANIUM TO CARBON RATIO OF ABOUT 8. WITH TITANIUM TO CARBON RATIOS OF BETWEEN 4 TO 8, NICKEL MUST BE PRESENT WITHIN THE RANGE OF .5 TO 1 PERCENT. FOR OPTIMUM WELD-TOUGHNESS THE MAXIMUM TITANIUM TO CARBON RATIO IS ABOUT 4, EITHER WITH OR WITHOUT NICKEL. TO ACHIEVE THE DESIRED COMBINATION OF STRENGTH AND TOUGHNESS, THE MATERIAL IN HOT-BAND GAGE IS ANNEALED FOR A TIME AT TEMPERATURE TO ACHIEVE A HARDNESS OF AT LEAST 80 R6 AND PREFERABLY 82 TO 92 R6.

United States Patent Int. Cl. C22f 1/04 US. Cl. 148-12 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a stainless steel and a method for producing the same, whereby material particularly adapted for use in structural applications such as the manufacture of cargo boxes is achieved. Specifically, the material is characterized by an improved combination of strength and toughness that is achieved by producing hotband material having a substantially martensitic microstructure of a composition consisting of .10 max. percent carbon, 2 max. percent manganese, 1 max. percent nickel, 9.5 to 13.5 percent chromium, and the balance iron. This material has a maximum titanium to carbon ratio of about 8. With titanium to carbon ratios of between 4 to 8, nickel must be present within the range of .5 to 1 percent. For optimum weld-toughness the maximum titanium to carbon ratio is about 4, either with or without nickel. To achieve the desired combination of strength and toughness, the material in hot-band gage is annealed for a time at temperature to achieve a hardness of at least 80 R and preferably 82 to 92 R For the purpose of satisfying various structural applications, a low-cost stainless steel having a combination of high strength and toughness, While exhibiting satisfactory formability, corrosion resistance and weldability, is desired. Such a material is particularly adapted to the manufacture of cargo boxes for ocean shipping. More specifical- 1y, steels for such structural applications are required to be readily formable and weldable without preor postheat treatment. The strength requirements vary with gage but generally such material should have a minimum tensile strength of 65,000 p.s.i. in combination with a minimum yield strength of 45,000 p.s.i., while exhibiting an elongation in 2 inches of a minimum of 20 percent. The material should possess good notch toughness and have at heavy gages a ductile-to-brittle impact transition temperature below F. The material must not only be weldable but must exhibit Weld toughness. Weld toughness is essential because structures such as cargo boxes are subjected at the joints to impact loads during service, and improper welding or inadequate weld properties can result in cracking at any notch defects. The corrosion-resistance requirements are not especially stringent; however, the corrosion resistance must be at least sufiicient to make possible the use of low-cost paint systems. Conventional low-alloy high-strength steels used for these particular applications, prior to the present invention, re-

3,778,316 Patented Dec. 11, 1973 Still another object of the invention is to provide a stainless steel having a good combination of high strength and toughness, said material being weldable without preor post-heat treatment and said weld heat-affected zone being characterized by good toughness and thus good resistance to cracking upon exposure to impact loads during service.

Yet another object of the invention is to provide a method for producing stainless steel characterized by a combination of high strength and toughness, with good Welding properties, by producing as-hot-rolled material having a substantially martensitic microstructure, and thereafter annealing said material for a time at temperature to achieve a hardness of at least 80 R and preferably within the range of 82 to 92 R These and other objects of the invention, as well as a complete understanding thereof, will be apparent from the following description and examples of the invention.

=In accordance with the present invention, a steel of the following composition in weight percent is provided:

Carbon .10 max.

Manganese 2 max.

Nickel lmax.

Chromium 9.5 to 13.5.

Titanium Max. 8 times carbon percent. Iron Balance.

With titanium to carbon ratios of between 4 to 8, nickel should be present within the range of .5 to 1 percent. This is necessary to obtain sufficient martensite in the steel for the purposes of the invention. The relatively high titanium content within this range combines with carbon to promote ferrite, and thus nickel is required to counter quired extensive surface preparation and special protec tive paints, which of course is avoided by using stainless steel produced in accordance with the present invention.

It is, accordingly, the primary object of the present thiseifect of high titanium.

Within the above-stated broad range, the following preferred ranges may be employed:

Carbon .03 to .08.

Manganese lmax.

Silicon .5 max.

Nickel lmax.

Chromium 10.5 to 12.5. Titanium Max. 4 times percent carbon. Nitrogen .03 max.

Iron Balance.

at atimgat temperature to achieve a hardness ofat least about R and preferably 82 to 92 R It has been found, as will'be demonstrated by specific examples reported hereinafter, that by annealing to a hardness within this range the above-listed stainless steel compositions will exhibit the required 'minimum strength. Specifically,

minimum tensile strengths of 65,000 p.s.i. in combination with minimum yield strengths on the order of 45,000 p.s.i. with a minimum elongation in 2 inches of 20 percent are produced. As will be demonstrated by the specific examples reported hereinafter, the substantially martensitic structure prior to annealing is necessary to achieve the'desired toughness in combination with high strength.

The required substantially martensitic hot-band microstructure is achieved by providing stainless steel within the composition limits as reported hereinabove. By adhering to these composition limits, and particularly by adhering to the recited titanium to carbon ratio, after hot rolling the as-hot-rolled material is characterized by a substantially matensitic microstructure, and consequently upon annealing it exhibits the required toughness.

To establish the criticality of the martensitic microstructure in the practice of the invention in achieving the required toughness, a steel was produced within the composition limits of the invention, except that the titanium to carbon ratio was 12 to 1 and thus provided a ferritic hot band. Upon annealing, the steel exhibited an impact transition temperature of 50 R, which signifies poor notch toughness.

To specifically demonstrate the present invention, four melted.

Not detectable when analyzed.

The compositions of Table I were processed in the conventional manner from ingot to hot-band gage. Materials were hot rolled, at a temperature of about 2100 F., to a hot-band gage of about .250 inch. Samples of the materials in hot-band gage were then subjected to the various annealing treatments as listed in Table H.

To illustrate annealing practices both within and without the scope of the invention and to establish the relationship between the annealed hardness of steel produced in accordance with the present invention and the required strength levels, various stainless steel compositions, within the composition range of the invention, were melted and subjected to various anneals as reported in Table III. The hardness values for each of these samples is recorded for comparison with the yield and tensile strength of each sample. From comparision of the hardness values with the tensile properties for a particular sample, it may be seen that the desired strength levels are achieved in all instances wherein the hardness values are at least 80 R and preferably within the range 82 to 92 R All of the samples exhibited a substantially martensitic microstructure in the hot-band gage, and thus, as reported in Table III, the toughness of the samples was excellent. However, as may be seen from Table IE, to achieve the required combination of strength, ductility and toughness, proper annealing to achieve a hardness of at least 80 R must be provided.

It will be understood, of course, that the specific annealing conditions required in the practice of the invention, particularly with regard to time and temperature, will depend on various factors, such as the mass and in particular the composition of the steel being treated. As may be seen from Table III, either strand or box annealing may be employed in the practice of the invention. All that is necessary in such practice is to determine the annealing conditions for a particular material that will achieve an annealed hardness of at least 80 R If this condition is obtained during annealing and if the composition of the It may be seen from the data reported in Table II that, by annealing to achieve hardness within the range above recited, the required strength levels are achieved.

All of these steels were characterized by a substantially martensitic microstructure in the as-hot-rolled condition, which resulted after annealing in excellent toughness as reported in Table H.

steel is within the ranges recited hereinabove to achieve a substantially martensitic microstructure in hot-band gage, then the required combination of high strength and toughness will be achieved.

The proper annealing treatment for the purposes of the invention is governed by the composition of the steel, as demonstrated by the data presented in Table IV.

TABLE III Longitudinal tensile-data 0.2% 01!- Percent Impact Hardset, yield Tensile elongatransitiln Heat ness, strength, strength, tion in temperature No. Condition R p.s.i. p.s.i. 2 inc 1025-- Box-annealed, 1,400 F 76 41,300 66,109 31.0 2216.-. Box-annealed, 1,475 F.... 78 40,400 66, 500 33.5 2216... Box-annealed, 1,400" F. 80 42, 600 69, 500 31.5 2326.-. Box-annealed, 1,500 F-.-. 80 47, 300 72,300 39. 5 2218...- Box-annealed, 1 475 F---- 82 54,300 77, 000 33.5 50. 2217... Strand-annealed, 1,500 F- 84 52, 200 80, 600 23.5 Below 70. 2232... Box-annealed 1 400 F...- 86 53, 600 76, 200 26.0 Below -80. 2227--- Strand-annea1ed,1,600 F. as ,500 21.0 Below -eo. 2224.-- Box-annealed, 1,400" F....' 90 65, 85,300 26.0 D0. 2218 ..do 93 72, 500 87,900 25. 0

TABLE IV Non-nickel-bearing steels Hardness Hardness (Rb) Material Condition Condition Heat 1025 5.2 Ti/G.

As-hot-rolled Strand-annealed-- At 1,400 F---.-. At 1,

At 1, Box-annealed At 1,400 Pu---- At 1,500 F......-

Nickel-bearing steels Titanium/carbon ratio below 4 Hard- Titanium/carbon ratio of 4 to 7 Hagg; Titanium/carbon ratio of 7 to 8 flgrecsl ness 11 Material Material Condition (Rb) Material (R Material Condition Condition 80 2.9 Ti/C.

As-hot-rolled... Strand-annealed 400 Fun...

Box-

At 1, At 1,426" F--..-.- At 1,475 F... At 1,500 F-.-......

The nickel content of the steel and the titanium to carbon ratio are particularly important, because these factors control the amount of martensite in proportion to ferrite in the steel and its tempering behavior. In general, the lower the nickel content and the higher the titanium content, the higher will be the ferrite content. In addition, the presence of nickel in more than a residual amount retards softening of the martensite in the steel during annealing. With nickel-bearing steels, wherein nickel is present in more than a residual amount, the annealing temperature required to achieve the desired strength levels depends primarily upon the specific ferrite-martensite balance, which is in turn determined by the titanium to carbon ratio. More specifically, for nickel-bearing steels with a titanium to carbon ratio below 4 box annealing at a temperature within the range of 1400 to 1500 R, which is near the lower critical temperature of the steel, is required. The nickelbearing steels with titanium to carbon ratios of about 4 to 7 contain more ferrite and thus to achieve the desired strength they must be box annealed at a lower temperature within the range of 1200 to 1400 F. Nickel-bearing steels having titanium to carbon ratios between about 7 and 8 contain substantial ferrite, and strand annealing within the temperature range of 1500 to 1700 F. is required. For these high-titanium steels the rapid softening due to the increased ferrite content precludes the use of box annealing to achieve the desired strength level. Nonnickel-bearing steels soften more rapidly as compared to nickel-containing steels of the invention. Consequently, to obtain the required strength levels either box appealing at 1200 to 1400 F. or strand annealing at 1200 to 1600 F. should be used. Non-nickel-bearing steels are considered to be those having a nickel content no greater than a residual amount, which is typically a maximum nickel content of about .25 percent.

TABLE V Impact transition temperature in Material Ti to G heat-affected Heat N 0. ratio zone, F.

Table V shows that to achieve weld toughness, the titanium to carbon ratio should be at the above-stated preferred maximum of about 4. It should be understood, however, that all the steels produced in accordance with the invention are readily weldable without preor post-heat treatment and have satisfactory weld-formability. For example, butt welds can be bent degrees without cracking. In Table V, the steels reported had compositions within the limits of the invention, as recited herein, except for the titanium to carbon ratio for material Heat. No. 128392.

It is understood that other adaptations and modifications of the invention may be made by those skilled in the art without departing from the scope and spirit of the appended claims.

We claim:

1. A method for producing a stainless steel characterized by an improved combination of strength and toughness comprising producing hot-band material having a substantially martensitic microstructure by hot-rolling to hotband gage a stainless steel consisting essentially of, in weight percent, .10 max. carbon, 2 max. manganese, 1 max. nickel, 9.5 to 13.5 chromium, max. titanium 8 times percent carbon, with nickel being present within the range of .5 to 1 when titanium is present in an amount greater than 4 times the carbon content, and the balance iron and incidental impurities, an annealing said hot-band material to a hardness of at least about 80 to 92 R by strand annealing at a temperature within the range of 1200 to 1600 F. when the nickel content of said stainless steel is not above a residual amount; box-annealing at a temperature Within the range of 1400 to 15 00 F. when the nickel content of said stainless steel is above a residual amount and the titanium content is less than 4 times the carbon content; box-annealing at a temperature within the range of 1200 to 1400 F. when the nickel content of said stainless steel is above a residual amount and the titanium content is within the range of 4 to 7 times the carbon content; and strand-annealing at a temperature within the range of 1500 to 1700 F. when the nickel content of said stainless steel is above a residual amount and the titanium content is within the range of about 7 to 8 times the carbon content.

2. The method of claim 1 wherein when the nickel content of said stainless steel is not above a residual amount, said hot-band material is box-annealed at a temperature within the range of 1200 to 1400 F.

References Cited UNITED STATES PATENTS 727.787 5/1903 Grimm 14812.1 2,120,319 6/1938 Wilson 14812 2,666,003 1/1954 Doughtery 148--12 2,808,353 10/1957 Leflingwell 14812 3,128,211 I 4/1964 Waxweiler 148135 3,141,800 7/1964 Reichenbach 148-12 3,373,015 Allen 148-135 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 

